High availability security device

ABSTRACT

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for processing a first plurality of packets using one or more processors and maintaining one or more flow records associated with the first plurality of packets, and processing a second plurality of packets without maintaining flow records associated with the second plurality of packets and allowing the second plurality of packets to pass to one or more destinations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/852,902, entitled HIGH AVAILABILITY SECURITY DEVICE, filedAug. 9, 2010, which is incorporated herein by reference for allpurposes, which claims priority to U.S. Provisional Application No.61/249,130, entitled HIGH AVAILABILITY SECURITY DEVICE, filed Oct. 6,2009, which is incorporated herein by reference for all purposes.

BACKGROUND

Security devices can be used to protect a computer network fromunauthorized, malicious or disruptive users. Examples of securitydevices include firewalls, Intrusion Detection Systems (IDSs), IntrusionPrevention Systems (IPSs), Unified Threat Management (UTM) systems, andthe like.

Some security devices are flow-based (also known as stateful securitydevices). Flow-based security devices typically store informationregarding flows associated with received packets. Flow-based securitydevices can perform security processing (for example, determiningwhether to drop a packet or allow it to pass) using the storedinformation regarding flows. Typically, a flow-based security devicestores information regarding flows in a flow table (also known as asession table or a state table).

Flow-based security devices are capable of tracking a limited number offlows. The number of flows is limited by, for example, the amount ofmemory available to a flow table, the processing power of the securitydevice (for instance, a Central Processing Unit (CPU) speed), and thelike. Flow-based security devices are not capable of handling new flowswhen their flow tables are saturated (when they lack capacity to storeinformation on additional flows). Consequently, a flow-based securitydevice will cause a denial of service (DoS) to resources protected bythe flow-based security device when its flow table is saturated.

Attackers can attempt DoS attacks on flow-based security devices byattempting to saturate the flow table. For example, attackers can send alarge number of illegitimate packets, sometimes referred to as a flood.A flow-based security device will attempt to store flow information forall of the packets in the flood, and consequently, the flow table willbecome saturated if it does not have sufficient capacity. Examples offlood attacks include Transmission Control Protocol (TCP) SYN floods,User Datagram Protocol (UDP) floods, and Internet Control MessageProtocol (ICMP) floods.

SUMMARY

In general, one aspect of the subject matter described in thisspecification can be embodied in a method that includes a methodcomprising processing a first plurality of packets using one or moreprocessors and maintaining one or more flow records associated with thefirst plurality of packets, and processing a second plurality of packetswithout maintaining flow records associated with the second plurality ofpackets and allowing the second plurality of packets to pass to one ormore destinations, where processing the second plurality of packetsincludes: receiving a second packet; determining not to maintain a flowrecord for a second flow associated with the second packet; and allowingthe second packet to pass to one of the one or more destinations. Otherembodiments of this aspect include corresponding systems, apparatus, andcomputer program products.

These and other embodiments can optionally include one or more of thefollowing features. Processing the first plurality of packets includesreceiving a first packet and, based on first header information of thefirst packet, determining whether a flow record exists for a first flowassociated with the first packet, where the first flow associated withthe first packet comprises a sequence of one or more packets received towhich the first packet belongs. Determining that a flow record for thefirst flow associated with the first packet does not exist. Creating anew flow record for the first flow associated with the first packet.Determining whether to allow the first packet to pass to one of the oneor more destinations. Determining whether a flow record exists for afirst flow associated with the first packet includes determining whetherthe flow record is present in a flow table; and wherein creating a newflow record for the first flow associated with the first packet includesadding the new flow record to the flow table. Creating a new flow recordfor the first flow associated with the first packet includes identifyingan application associated with the first packet. The first headerinformation includes two or more of: a source port, a destination port,a source address, a destination address, and a protocol. Determining notto maintain a flow record for a second flow associated with the secondpacket includes determining that the second packet is associated with apeer-to-peer networking application. Determining not to maintain a flowrecord for a second flow associated with the second packet includesdetermining that the second packet is associated with Domain NameService (DNS) traffic. Determining that the second packet is associatedwith DNS traffic includes determining that the destination address ofthe second packet or the source address of the second packet correspondsto a trusted DNS server. The second packet can be a TCP packet or a UDPpacket.

In general, another aspect of the subject matter described in thisspecification can be embodied in a method that includes a methodcomprising processing a first plurality of packets using one or moreprocessors and maintaining one or more flow records associated with thefirst plurality of packets, and processing a second plurality of packetswithout maintaining flow records associated with the second plurality ofpackets and allowing the second plurality of packets to pass to one ormore destinations, where processing the second plurality of packetsincludes: receiving a second packet; determining not to maintain a flowrecord for a second flow associated with the second packet; and blockingthe second packet. Other embodiments of this aspect includecorresponding systems, apparatus, and computer program products.

Particular embodiments of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages. Flow table saturation can be prevented in aflow-based security device, and thus denial of service can be prevented.Consequently, the flow-based security device has higher availability. Afilter in a flow-based security device can allow certain packets to passwithout adding flow records for those packets to the flow table. Aflow-based security device can handle a larger volume of traffic givenfixed resources, for example, memory and processing power.

The details of one or more embodiments of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of theinvention will become apparent from the description, the drawings, andthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example network topology 100.

FIG. 2 is a flow diagram of an example technique 200 for processingpackets at a security device.

FIG. 3 is a block diagram of an example security device 300.

FIG. 4 illustrates a structure of an example flow table 400.

FIG. 5 is a flow diagram of an example technique 500 for processingpackets at a security device and improving availability of the securitydevice.

FIG. 6 is a schematic diagram of a generic computer system 600.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 shows an example network topology 100. The example networktopology 100 includes workstations 102, a security device 104, a router106, and a Domain Name Service (DNS) server 110. The router 106 isconnected to a network 108, for example, the Internet.

The network topology shown in FIG. 1 is merely an example. The networkcan be configured in various ways. For example, the network can includeadditional security devices, the security device 104 can be integratedwith the router 106, and so on. In addition to the workstations 102,various devices can communicate using the security device 104, forexample, mobile devices, Voice over IP (VOIP) phones, and the like.Other network topologies are possible including those that include morethan one network, wide area networks, local area networks orcombinations of both with or without distributed topologies (forexample, topologies other than client-server topologies). Networks canbe completely private and not connected to the Internet. For example,some government networks are extensive and yet not connected to theInternet, and the security device 104 can be used in those networks.

The security device 104 monitors packets communicated on the network andperforms security processing. Security processing includes, for example,blocking packets associated with an attempted network securityintrusion, detecting and removing viruses, Data Leakage Prevention(DLP), and so on. The security device includes an interface 112, afiltering engine 114, and a flow-based security engine 116.

The interface 112 includes, in some implementations, one or moreEthernet ports for receiving and sending packets. In general, thepackets are Transmission Control Protocol/Internet Protocol (TCP/IP)packets. Alternatively, the packets are User Datagram Protocol/InternetProtocol (UDP/IP) packets, Real Time Transport Protocol (RTP) packets,or other types of packets. The packets include header information andcontent information. For example, the header information of TCP/IPpackets includes a five tuple including source address, destinationaddress, source port, destination port, and protocol.

The filtering engine 114 determines whether the security device 104 willprocess packets using the flow-based security engine 116. In someimplementations, the filtering engine 114 inspects the headerinformation of received packets. Based on the header information, thefiltering engine determines whether the security device 104 will processpackets using the flow-based security engine 116. If the security devicedoes not process a packet using the flow-based security engine 116, thenthe security device does not maintain a flow record for the flowassociated with the packet.

In some implementations, the security device 104 allows packets to passif it decides not to process those packets using the flow-based securityengine 116. In other implementations, the security device 104 determineswhether to allow packets to pass or to block those packets based on, forexample, the header information of those packets.

A flow is a sequence of one or more packets communicating informationbetween a source and a destination (for example, from a workstation 102to the DNS server 110). A flow typically includes packets going in onedirection (source to destination or the reverse) but in someimplementations can include packets going in both directions. Forexample, in some implementations, a flow is a TCP connection or one sideof a TCP connection (going only from a source to a destination). Inanother example, a flow is a sequence of UDP packets. Although UDP isconnectionless, the flow based security engine 116 can identify a UDPflow based on, for example, header information of the UDP packets.

In some implementations, the flow-based security engine includes a flowtable 118. The flow table 118 includes flow records for flows associatedwith packets. A flow record includes, for example, a five tuple ofheader information and other flow information. Flow informationincludes, for example, security policy information (for example, policyto apply to the flow, such as firewall policy, IPS policy, and so on),encryption parameters, address translation parameters, bookkeepinginformation, statistics, network policy information (for example,relating to connection time outs, time billing, bandwidth usage), areference to one or more applications associated with the flow, and soon. Flow tables are discussed further in reference to FIG. 4 which is adiagram of an example flow table.

The flow-based security engine processes a packet by determining whethera flow record for a flow associated with that packet exists, forexample, in a flow table 118. In some implementations, where the flow isone side of a TCP connection, the flow-based security engine uses, forexample, the five tuple of source address, destination address, sourceport, destination port, and protocol (TCP in this case) to determine ifa flow record in the flow table 118 has a matching record.

If the flow record exists, then the flow-based security engine 116performs security processing using the packet and any flow informationincluded in the flow record. If the flow record does not exist, then theflow-based security engine 116 creates a new flow record and, ingeneral, adds the new flow record to the flow table 118. The flow-basedsecurity engine 116 determines whether to add any flow information tothe flow record (for example, instructions to drop all packetsassociated with the flow). The flow-based security engine 116 thenperforms security processing for the packet.

Although only three workstations 102 are shown, potentially largenumbers of workstations 102 can be protected using the security device104. In that case, the workstations can generate a large number of flowsand saturate the flow table 118. For example, if the flow table 118 isonly able to hold 1 million flow records, and the workstations 102simultaneously make one million DNS requests to the DNS server 110, thenthe flow table 118 will saturate. New flows, for example, from thenetwork 108 to the workstations 102, will be blocked by the securitydevice 104 because it cannot process them.

However, if the DNS server 110 is trusted (for example, if it is closelycontrolled), then the one million DNS requests do not have to beprocessed using the flow-based security engine 116. The filtering engine114 can be configured so that, for any packets bound from a workstation102 and destined to the DNS server 110, those packets are not processedusing the flow-based security system and are allowed to pass. Then, theone million DNS requests will not saturate the flow table 118 and thesecurity device 104 can process new flows using the flow-based securityengine 116.

Similarly, when a workstation 102 attempts to download a file using apeer-to-peer application (for example, eMule), the workstation 102 canrapidly create many new flows while trying to download the file frommany peer-to-peer clients over network 108. Thus, the flow table 118 canpotentially become saturated. However, the filtering engine 114 can beconfigured so that, for any packets bound from a workstation 102 to aknown peer-to-peer client, those packets are not processed using theflow-based security system 116. The security device 104 can either allowthose packets to pass or block them. Other types of inspection freescenarios are possible, such as those between trusted devices, trustedzones or other configurations.

FIG. 2 is a flow diagram of an example technique 200 for processingpackets at a security device. The security device can be, for example, afirewall, an IDS, an IPS, a UTM device, a device with functionality ofcombinations of these devices, or the like.

At step 202, a first plurality of packets are processed and one or moreflow records associated with the first plurality of packets aremaintained (step 202). Processing the first plurality of packetsincludes steps 204, 206, 208, 210, and 212. Processing the firstplurality of packets can be performed by a security device (for example,security device 104).

A first packet is received (step 204). The first packet includes headerinformation and content. The first packet can be from, for example, aworkstation (for instance, workstation 102) with a destination of aserver on the Internet (for example, to network 108).

A determination is made whether a flow record exists for a first flowassociated with the first packet (step 206). The first flow associatedwith the first packet comprises a sequence of one or more packetsreceived by the security device. The first packet belongs to thesequence of one or more packets. The sequence of one or more packetscommunicates information between a first source and a first destination.

In some implementations, where the first packet is a TCP/IP packet andthe first flow is one half of a TCP connection, a security device caninspect the header information of the first packet to determine a fivetuple of source address, destination address, source port, destinationport, and protocol (in this case, TCP). In some implementations, eachpacket of the first flow has the same five tuple. The security deviceuses the five tuple to determine whether a flow table (or other datastructure) includes a flow record for the first flow.

A determination is made that a flow record for the first flow does notexist (step 208). For example, in some cases, a security device searcheseach flow record in a flow table and does not find a flow record for thefirst flow. In another example, where a flow table including flowrecords is a hash table, the security device searches the hash table(for instance, using a hash of a five tuple) and does not find a flowrecord for the first flow. Various techniques for determining whether aflow record for the first flow exists are possible.

A new flow record for the first flow is created (step 210). In someimplementations, the new flow record is added to a flow table. In someimplementations, a security device determines flow information (forexample, whether to allow or drop packets associated with the firstflow) and adds the flow information to the new flow record.

A determination is made whether to allow the first packet to pass to itsdestination (step 212). A security device can use various techniques orcombinations of techniques to make the determination. In someimplementations, the security device is configured with a list ofdestination addresses (or addresses and ports) or source addresses (oraddresses and ports) to block. In some implementations, the securitydevice is configured with a list of applications and instructions toblock any packets associated with those applications. In someimplementations, the security device identifies an applicationassociated with the first packet (or attempts to identify theapplication) and determines whether to allow the first packet to passbased on the application.

If a determination is made to allow the first packet to pass, then thefirst packet is transmitted (for example, to a router such as router106) (step 213). Else, the packet is dropped, logged or otherwiseprocessed (214). In some implementations, a security device logs blockedpackets. In some implementations, the security device reports blockedpackets, for example, to a central management system.

In some implementations, the security device performs other securityprocessing using the first packet. For example, the security device canremove a virus (or an infected portion of the first packet) from thefirst packet. In another example, the security device can removeconfidential information (for example, a credit card number or a socialsecurity number) from the first packet.

A second plurality of packets is thereafter processed withoutmaintaining flow records associated with the second plurality of packets(allowing the second plurality of packets to pass) (step 215).Processing the second plurality of packets includes steps 216, 218, and220.

A second packet is received (step 216). For example, the second packetcan be a UDP packet from a workstation (for instance, workstation 102)to a DNS server (for instance, DNS server 110). In another example, thesecond packet can be a TCP SYN packet from a workstation (for instance,workstation 102) attempting to open a TCP connection using apeer-to-peer file sharing application (for instance, over network 108).

A determination is made (e.g., based on header information) not tomaintain a flow record for a second flow associated with the secondpacket (step 218). In some implementations, a security device comparessome header information (for example, the five tuple of source addressand port, destination address and port, and protocol) against a listspecifying types of packets for which the security device is not tomaintain flow records.

In some implementations, the security device includes a flow-basedsecurity engine (for example, security device 104 includes flow-basedsecurity engine 116). In some implementations, the security devicedetermines not to process the second packet using the flow-basedsecurity engine.

The second packet is allowed to pass to its destination (step 220). Insome implementations, a security device allows all packets that it doesnot maintain flow records for to pass. In some implementations, thesecurity device determines whether to allow the second packet to pass orto block the second packet. For example, the security device can comparesome of the header information of the second packet against a listspecifying whether to allow certain types of packets or block certaintypes of packets. In another example, the security device performssecurity processing (for instance, scanning for viruses, or the like)without maintaining a flow record for the second flow. In a furtherexample, the security device compares some of the header information ofthe second packet against a list specifying whether to allow or blockcertain IP addresses, port numbers, or both.

Because the security device does not maintain a flow record for thesecond flow, the security device has more memory (or other resources)available for other flows (for example, the first flow associated withthe first packet). This can prevent the security device from causing adenial of service (when a flow table is saturated or the security deviceis otherwise low on resources such as processing power) and thusincreases the availability of the security device.

For example, in some cases, a first amount of memory allocated at thesecurity device for a flow table is smaller than a second amount ofmemory required for flow records in the flow table for flows associatedwith the both the first plurality of packets and the second plurality ofpackets. In those cases, if the security device maintained flow recordsfor both the first plurality of packets and the second plurality ofpackets, the flow table will saturate and the security device will beunable to process packets associated with new flows. However, if thesecurity device does not maintain flow records for the second pluralityof packets, and if the first amount of memory is greater than a thirdamount of memory required for flow records in the flow table for flowsassociated with the first plurality of packets (and not the secondplurality of packets), then the flow table will not saturate and thesecurity device will be available to process packets associated with newflows.

FIG. 3 is a block diagram of an example security device 300. Thesecurity device 300 is one example of a security device configured toexecute the technique 200 described in reference to FIG. 2.

The security device 300 includes an interface 302 for incoming andoutgoing packets, a filtering engine 304, and a flow-based securityengine 306. The interface 102 can include, for example, one or moreEthernet ports for receiving and sending packets. When the securitydevice 300 receives packets, the filtering engine 304 determines whetherto process the packets using the flow-based security engine 306. Thefiltering engine 304 makes that determination based on, for example,header information in the packets.

The flow-based security engine includes a multi-mode classificationengine 308, a flow table 316, and a security engine 318. The multi-modeclassification engine 308 analyzes packets and attempts to identifyapplications associated with packets. The multi-mode classificationengine can also attempt to classify other attributes of the packets.

In some implementations, the multi-mode classification engine 308includes plural modes for classifying incoming packets that are updatedas more packets are received. Multi-mode classification engine 308includes a header classification engine 310 and a signature matchingengine 312.

The header classification engine 310 uses header information of areceived packet to classify the packet for further processing. Forexample, in some implementations the header classification engine 310identifies a five-tuple of information about each packet, includingprotocol, source address, source port, destination address, anddestination port. Other techniques for header classification arepossible.

The signature matching engine 312 classifies packets for furtherprocessing based on signatures detected, for example at the applicationlevel. The signature matching engine 312 uses the contents of receivedpackets. In general, the signature matching engine 312 includes asignature database 314 that is typically local. In some implementations,the signature database 314 is distributed onto devices remote from thesecurity device 300. Signature matching can include fixed patternmatching, regular expression matching, and specialized matchingperformed by, for example, specialized software to detect complexapplication behavior patterns or a combination of the above. In someimplementations, the multi-mode classification engine 304 performsTransmission Control Protocol (TCP) reassembly before performingsignature matching or other analysis to determine an applicationassociated with received packets.

The flow table 316 includes flow records associated with flows. The flowrecords in the flow table 316 include flow information. In someimplementations, flow information includes information identifying anapplication associated with a flow. The signature matching engine 312attempts to identify applications associated with flows and when it issuccessful it updates the flow table 316.

The security engine 318 performs security processing. Securityprocessing includes, for example, blocking packets associated with anattempted network security intrusion, detecting and removing viruses,Data Leakage Prevention (DLP), and so on. If the security engine 318determines that a flow should be blocked, the security engine 318 canadd an indication to a corresponding flow record in the flow table 316.

FIG. 4 illustrates a structure of an example flow table 400. The rows402, 404, and 406 of the flow table 400 represent flow recordsassociated with flows.

The first flow record 402 includes an indexing key 408. A securitydevice can use the indexing key 408 to store and retrieve the flowrecord 402 when it receives a packet associated with the flow. In someimplementations, the indexing key 408 is a hash key (for example, basedon header information) and the flow table 400 is implemented as a hashtable. The flow record 402 also includes flow information 410.

Flow information 410 includes, for example, an indication 412 of whetherto allow or deny packets associated with the flow, a reference 414 to anapplication (for example, peer-to-peer networking application eMule)associated with the flow, and a session timer 416. The session timer 416indicates how long a session (for example, a TCP session) has been in acertain state (for example, whether a TCP connection is open orcurrently being established). In some implementations, a securitydevices removes flow records from the flow table 400 when the sessiontimer indicates that a session has been in a certain state (for example,establishing a TCP connection) for a time greater than a threshold time.This can reduce the number of flow records in the flow table and thusimprove availability of the security device.

FIG. 5 is a flow diagram of an example technique 500 for processingpackets (e.g., at a security device). The technique will be describedwith reference to a security device, though more than one or othercomponent configurations are possible.

A packet is received (step 502). A determination is made (e.g., based onheader information in the packet) whether to process the packet (e.g.,using a flow-based security system) (step 506).

If it is determined not to process the packet (e.g., using a flow-basedsecurity system), the packet is allowed to pass to its destination oroptionally a determination is made whether to allow the packet or blockthe packet (step 508). A security device can make that determinationusing, for example, the header information of the packet or the content,but not using a flow table. Based on that determination, the packet isallowed (step 510) or blocked (step 512).

If the security device determines to process the packet (e.g., using theflow-based security system), a determination is made whether a flowrecord for a flow associated with the packet exists (step 514). If aflow record does not exist, a flow record is created (step 516) andoptionally flow information is added to the flow record (step 518).Security processing of the packet is initiated (step 520). Adetermination of whether to allow the packet or to block the packet ismade (step 522). Based on that determination, the packet is allowed(step 510) or blocked (step 512).

FIG. 6 is a schematic diagram of a generic computer system 600. Thesystem 600 can be used for practicing operations described inassociation with the techniques 200 and 500 described above. The system600 can include a processor 610, a memory 620, a storage device 630, andinput/output devices 640. Each of the components 610, 620, 630, and 640are interconnected using a system bus 650. The processor 610 is capableof processing instructions for execution within the system 600. Suchexecuted instructions can implement one or more components of securitydevice 300, for example. In some implementations, the processor 610 is asingle-threaded processor. In other implementations, the processor 610is a multi-threaded processor. The processor 610 is capable ofprocessing instructions stored in the memory 620 or on the storagedevice 630 to display graphical information for a user interface on theinput/output device 640.

The memory 620 is a computer readable medium such as volatile or nonvolatile that stores information within the system 600. The memory 620can store data structures representing flow tables, for example. Thestorage device 630 is capable of providing persistent storage for thesystem 600. The storage device 630 can be a floppy disk device, a harddisk device, an optical disk device, or a tape device, or other suitablepersistent storage means. The input/output device 640 providesinput/output operations for the system 600. In some implementations, theinput/output device 640 includes a keyboard and/or pointing device. Invarious implementations, the input/output device 640 includes a displayunit for displaying graphical user interfaces.

The input/output device 640 can provide input/output operations for asecurity device. For example, the input/output device 640 can includeEthernet ports or other networking ports to communicate with computernetworks.

Various implementations of the systems and techniques described here canbe realized in digital electronic circuitry, integrated circuitry,specially designed ASICs (application specific integrated circuits),computer hardware, firmware, software, and/or combinations thereof.These various implementations can include implementation in one or morecomputer programs that are executable and/or interpretable on aprogrammable system including at least one programmable processor, whichmay be special or general purpose, coupled to receive data andinstructions from, and to transmit data and instructions to, a storagesystem, at least one input device, and at least one output device.

These computer programs (also known as programs, software, softwareapplications or code) include machine instructions for a programmableprocessor, and can be implemented in a high-level procedural and/orobject-oriented programming language, and/or in assembly/machinelanguage. As used herein, the terms “machine-readable medium” and“computer-readable medium” refer to any computer program product,apparatus and/or device (e.g., magnetic discs, optical disks, memory,Programmable Logic Devices (PLDs)) used to provide machine instructionsand/or data to a programmable processor.

To provide for interaction with a user, the systems and techniquesdescribed here can be implemented on a computer having a display device(e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor)for displaying information to the user and a keyboard and a pointingdevice (e.g., a mouse or a trackball) by which the user can provideinput to the computer. Other kinds of devices can be used to provide forinteraction with a user as well; for example, feedback provided to theuser can be any form of sensory feedback (e.g., visual feedback,auditory feedback, or tactile feedback); and input from the user can bereceived in any form, including acoustic, speech, or tactile input.

In some implementations, users can interact with a security device usinga computer on a network. For example, a user on a personal computer caninteract with a security device connected on the network using a webbrowser, a Secure Shell (SSH) client, a telnet client, or the like. Invarious implementations, a user can interact with a security deviceusing a computer connected to the security device on a serial port, forexample, a Recommended Standard 232 (RS-232) port.

The systems and techniques described here can be implemented in acomputing system that includes a back end component (e.g., as a dataserver), or that includes a middleware component (e.g., an applicationserver), or that includes a front end component (e.g., a client computerhaving a graphical user interface or a Web browser through which a usercan interact with an implementation of the systems and techniquesdescribed here), or any combination of such back end, middleware, orfront end components. The components of the system can be interconnectedby any form or medium of digital data communication (e.g., acommunication network). Examples of communication networks include alocal area network (“LAN”), a wide area network (“WAN”), and theInternet.

While this specification contains many implementation details, theseshould not be construed as limitations on the scope of the invention orof what may be claimed, but rather as descriptions of features specificto particular implementations of the invention. Certain features thatare described in this specification in the context of separateimplementations can also be implemented in combination in a singleimplementation. Conversely, various features that are described in thecontext of a single implementation can also be implemented in multipleimplementations separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular implementations of the invention have been described.Other implementations are within the scope of the following claims. Forexample, the actions recited in the claims can be performed in adifferent order and still achieve desirable results.

What is claimed is:
 1. A method implemented by a security device,comprising: filtering a plurality of packets into a filtered firstplurality of packets and a filtered second plurality of packets based onheader information of each of the plurality of packets, wherein thefiltering the plurality of packets comprises: filtering the plurality ofpackets into the filtered second plurality of packets in response to theheader information of the corresponding packets being associated withDomain Name Service (DNS) traffic or a peer-to-peer networkingapplication, wherein the DNS traffic relates to communications with atrusted DNS server, the trusted DNS server being a closely-controlledDNS server, and wherein the peer-to-peer networking application relatesto a file download using the peer-to-peer networking application;processing the filtered first plurality of packets using one or moreprocessors of the security device and maintaining one or more flowrecords associated with the filtered first plurality of packets, andprocessing the filtered second plurality of packets using one or moreprocessors of the security device without maintaining flow recordsassociated with the filtered second plurality of packets and allowingthe filtered second plurality of packets to pass to one or moredestinations, where processing the filtered second plurality of packetsincludes: receiving a second packet; determining not to maintain a flowrecord for a second flow associated with the second packet; and allowingthe second packet to pass to one of the one or more destinations.
 2. Themethod of claim 1 wherein processing the filtered first plurality ofpackets includes receiving a first packet and, based on first headerinformation of the first packet, determining whether a flow recordexists for a first flow associated with the first packet, where thefirst flow associated with the first packet comprises a sequence of oneor more packets received to which the first packet belongs.
 3. Themethod of claim 2 further comprising: determining that a flow record forthe first flow associated with the first packet does not exist; creatinga new flow record for the first flow associated with the first packet;and determining whether to allow the first packet to pass to one of theone or more destinations.
 4. The method of claim 3 wherein determiningwhether a flow record exists for a first flow associated with the firstpacket includes determining whether the flow record is present in a flowtable; and wherein creating a new flow record for the first flowassociated with the first packet includes adding the new flow record tothe flow table.
 5. The method of claim 3 wherein creating a new flowrecord for the first flow associated with the first packet includesidentifying an application associated with the first packet.
 6. Themethod of claim 2 wherein the first header information includes two ormore of: a source port, a destination port, a source address, adestination address, and a protocol.
 7. The method of claim 1 whereindetermining not to maintain a flow record for a second flow associatedwith the second packet includes determining that the second packet isassociated with the peer-to-peer networking application.
 8. The methodof claim 1 wherein determining not to maintain a flow record for asecond flow associated with the second packet includes determining thatthe second packet is associated with the Domain Name Service (DNS)traffic.
 9. The method of claim 1, wherein the second packet is atransmission control protocol (TCP) packet.
 10. The method of claim 1,wherein the second packet is a user datagram protocol (UDP) packet. 11.A system comprising: a non-transitory computer-readable medium includinga program product; and one or more processors configured to interactwith the non-transitory computer-readable medium, execute the programproduct and perform operations comprising: filtering a plurality ofpackets into a filtered first plurality of packets and a filtered secondplurality of packets based on header information of each of theplurality of packets, wherein the filtering the plurality of packetscomprises: filtering the plurality of packets into the filtered secondplurality of packets in response to the header information of thecorresponding packets being associated with Domain Name Service (DNS)traffic or a peer-to-peer networking application, wherein the DNStraffic relates to communications with a trusted DNS server, the trustedDNS server being a closely-controlled DNS server, and wherein thepeer-to-peer networking application relates to a file download using thepeer-to-peer networking application; processing the filtered firstplurality of packets using one or more processors and maintaining one ormore flow records associated with the filtered first plurality ofpackets, and processing the filtered second plurality of packets withoutmaintaining flow records associated with the filtered second pluralityof packets and allowing the filtered second plurality of packets to passto one or more destinations, where processing the filtered secondplurality of packets includes: receiving a second packet; determiningnot to maintain a flow record for a second flow associated with thesecond packet; and allowing the second packet to pass to one of the oneor more destinations.
 12. The system of claim 11 wherein processing thefiltered first plurality of packets includes receiving a first packetand, based on first header information of the first packet, determiningwhether a flow record exists for a first flow associated with the firstpacket, where the first flow associated with the first packet comprisesa sequence of one or more packets received to which the first packetbelongs.
 13. The system of claim 12 wherein the operations furthercomprise: determining that a flow record for the first flow associatedwith the first packet does not exist; creating a new flow record for thefirst flow associated with the first packet; and determining whether toallow the first packet to pass to one of the one or more destinations.14. The system of claim 13 wherein determining whether a flow recordexists for a first flow associated with the first packet includesdetermining whether the flow record is present in a flow table; andwherein creating a new flow record for the first flow associated withthe first packet includes adding the new flow record to the flow table.15. The system of claim 13 wherein creating a new flow record for thefirst flow associated with the first packet includes identifying anapplication associated with the first packet.
 16. The system of claim 12wherein the first header information includes two or more of: a sourceport, a destination port, a source address, a destination address, and aprotocol.
 17. The system of claim 11 wherein determining not to maintaina flow record for a second flow associated with the second packetincludes determining that the second packet is associated with thepeer-to-peer networking application.
 18. The system of claim 11 whereindetermining not to maintain a flow record for a second flow associatedwith the second packet includes determining that the second packet isassociated with the Domain Name Service (DNS) traffic.
 19. The system ofclaim 11, wherein the second packet is a transmission control protocol(TCP) packet.
 20. The system of claim 11, wherein the second packet is auser datagram protocol (UDP) packet.
 21. A non-transitory computerprogram product, encoded on a computer-readable storage device, operableto cause data processing apparatus to perform operations comprising:filtering a plurality of packets into a filtered first plurality ofpackets and a filtered second plurality of packets based on headerinformation of each of the plurality of packets, wherein the filteringthe plurality of packets comprises: filtering the plurality of packetsinto the filtered second plurality of packets in response to the headerinformation of the corresponding packets being associated with DomainName Service (DNS) traffic or a peer-to-peer networking application,wherein the DNS traffic relates to communications with a trusted DNSserver, the trusted DNS server being a closely-controlled DNS server,and wherein the peer-to-peer networking application relates to a filedownload using the peer-to-peer networking application; processing thefiltered first plurality of packets using one or more processors andmaintaining one or more flow records associated with the filtered firstplurality of packets, and processing the filtered second plurality ofpackets without maintaining flow records associated with the filteredsecond plurality of packets and allowing the filtered second pluralityof packets to pass to one or more destinations, where processing thefiltered second plurality of packets includes: receiving a secondpacket; determining not to maintain a flow record for a second flowassociated with the second packet; and allowing the second packet topass to one of the one or more destinations.
 22. The program product ofclaim 21 wherein processing the filtered first plurality of packetsincludes receiving a first packet and, based on first header informationof the first packet, determining whether a flow record exists for afirst flow associated with the first packet, where the first flowassociated with the first packet comprises a sequence of one or morepackets received to which the first packet belongs.
 23. The programproduct of claim 22 wherein the operations further comprise: determiningthat a flow record for the first flow associated with the first packetdoes not exist; creating a new flow record for the first flow associatedwith the first packet; and determining whether to allow the first packetto pass to one of the one or more destinations.
 24. The program productof claim 23 wherein determining whether a flow record exists for a firstflow associated with the first packet includes determining whether theflow record is present in a flow table; and wherein creating a new flowrecord for the first flow associated with the first packet includesadding the new flow record to the flow table.
 25. The program product ofclaim 24 wherein creating a new flow record for the first flowassociated with the first packet includes identifying an applicationassociated with the first packet.
 26. The program product of claim 22wherein the first header information includes two or more of: a sourceport, a destination port, a source address, a destination address, and aprotocol.
 27. The program product of claim 21 wherein determining not tomaintain a flow record for a second flow associated with the secondpacket includes determining that the second packet is associated withthe peer-to-peer networking application.
 28. The program product ofclaim 21 wherein determining not to maintain a flow record for a secondflow associated with the second packet includes determining that thesecond packet is associated with the Domain Name Service (DNS) traffic.29. The program product of claim 21, wherein the second packet is atransmission control protocol (TCP) packet.
 30. The program product ofclaim 21, wherein the second packet is a user datagram protocol (UDP)packet.